Drill drives, used for oil and gas drilling operations in particular, usually have a drive shaft which drives a drill string. The drive shaft is supported by means of a bearing arrangement which serves to stabilize the drive shaft both in an axial position and in a radial position.
To take up and transmit the drive torque as well as the radial bending forces resulting from the drill string radial bearings can be provided. These can be cylindrical roller bearings and tapered roller bearings, the axial force component of which can counteract the additional axial bearing. To take up the forces of the drive shaft that act in the axial direction provision is usually made for axial bearings.
Due to the fact that in drilling for oil and gas the drilling progress as well as the wear of the drill head significantly depends on the load of the drill head, in addition to the abrasiveness of the ground material, it is essential for the operation of the drilling rig that the load on the drill head is known as precisely as possible. For this purpose, as is known, the drill head has e.g. sensors which measure the pressure imposed on the drill head and forward this via lines or other transmission means in the upward direction to the drillmaster. The transmission of the measured values from the drill head involves a considerable effort and is marked by additional sources of error.
Another possibility of measuring the load on the drill head is to install a load measuring cell below the rotary drive before the beginning of the drill string. Since the load measuring cell co-rotates with the drive shaft during operation it has to be equipped with an energy store in order to transmit the measured values to the drillmaster.
Furthermore, a contributory factor in the inaccuracy of the measurement are the prestressing forces prevailing in a bearing arrangement, through which the applied load exerted on the drive shaft deviates from the load on the drill head.